Thin film transistor and method for fabricating the same

ABSTRACT

A thin film transistor and a method for fabricating the same are disclosed, in which an offset region is affected or biased by a gate voltage to increase on-current, thereby improving on/off characteristic of a device. A first semiconductor layer is formed on a substrate, and insulating layer patterns are formed at both ends of the first semiconductor layer. A second semiconductor layer is formed on the first semiconductor layer and the insulating layer patterns. A gate insulating film is formed on the first and second semiconductor layers and the insulating layer patterns, and an active layer formed on the gate insulating film.

This application is a Divisional of application Ser. No. 09/199,161 filed Nov. 25, 1998, now U.S. Pat. No. 5,998,839.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device, and more particularly, a thin film transistor and a method for fabricating the same.

2. Background of the Related Art

Generally, a thin film transistor is used as a CMOS load transistor or a load transistor in an SRAM cell of 4M or 1M. The thin film transistor is also used as a switching device for switching a picture data signal in each pixel region in a liquid crystal display device.

Particularly, an off-current should be reduced and an on-current should increase when a PMOS thin film transistor is used as the load transistor in the SRAM cell. For this reason, power consumption of the SRAM cell may be reduced and its memory characteristic is increased, thereby obtaining the SRAM cell of high quality.

FIG. 1 is a sectional view of a thin film transistor of the background art. The thin film transistor includes an insulating layer 21, a gate electrode 22 a formed on the insulating layer 21, and a gate insulating film 24 formed on the insulating layer 21 including the gate electrode 22 a. A drain electrode D is formed on the gate insulating film 24 at a predetermined distance from the gate electrode 22 a, and a source electrode S is formed on the gate insulating film 24 to overlap the gate electrode 22 a and spaced apart from the drain electrode D. A channel region I and an offset region II are formed on the gate insulating film 24 between the source electrode S and the drain electrode D. The offset region II is formed between the drain electrode D and the gate electrode 22 a.

The method for fabricating the thin film transistor will be described with reference to FIGS. 2A to 2D. As shown in FIG. 2A, a first polysilicon layer 22 for a gate electrode of a bulk transistor is formed on the insulating layer or substrate 21. A photoresist is deposited on the first polysilicon layer 22 and then a mask pattern 23 is formed by exposure and development processes. Subsequently, the first polysilicon layer 22 is selectively removed by an etching process using the mask pattern 23 to form a gate electrode 22 a, as shown in FIG. 2B.

As shown in FIG. 2C, a gate insulating film 24 is deposited on the insulating layer 21 including the gate electrode 22 a. A second polysilicon layer 25 is then formed on the gate insulating film 24, which will be used to form the source and drain electrodes, an offset region and a channel region in the thin film transistor. Subsequently, a photoresist 26 is deposited on the second polysilicon layer 25 and then patterned by exposure and development processes.

As shown in FIG. 2D, a channel region and an offset region are defined by a photoresist pattern 26 a of the patterned photoresist 26. Source and drain impurity ions or dopants are then implanted into the exposed second polysilicon layer 25 using the photoresist pattern 26 a as a mask. Thus, a source electrode S is formed to partially overlap the gate electrode 22 a and a drain electrode D is formed at a predetermined distance from the gate electrode 22 a. The channel region I and the offset region II are formed between the source electrode S and the drain electrode D.

The above method for fabricating the thin film transistor has a problem that the on-current is small because the offset region is not affected by the gate voltage during the PMOS thin film transistor operation, thereby deteriorating reliability of the device.

SUMMARY OF THE INVENTION

An object of the present invention is to obviate at least the problems and disadvantages of the related art.

Another object of the present invention is to reduce the off current.

A further object of the present invention is to increase the on-current.

Still another object of the present invention is to provide a thin film transistor and a method for fabricating the same, in which an offset region is biased by a gate voltage to increase on-current, thereby improving on/off characteristic of a device.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a thin film transistor according to the present invention includes a first semiconductor layer formed on a substrate, insulating layer patterns formed at both ends of the first semiconductor layer, a second semiconductor layer formed on the first semiconductor layer and the insulating layer patterns, a gate insulating film formed on the first and second semiconductor layers and the insulating layer patterns, and an active layer formed on the gate insulating film.

In another aspect, a method for fabricating the thin film transistor according to the present invention includes the steps of forming a first semiconductor layer on a substrate, forming insulating layer patterns on both ends of the first semiconductor layer, forming a second semiconductor layer on the first semiconductor layer and the insulating layer patterns, forming a gate insulating film on the first and second semiconductor layers and the insulating layer patterns, and forming an active layer on the gate insulating film.

The present invention can be achieved in parts or a whole by a semiconductor device, comprising: a first conductive layer formed on a substrate; an offset spacer formed on the first conductor; a second conductive layer formed on the first conductive layer and the offset spacer; a first insulation layer formed over the substrate, the first and second conductive layers and the offset spacer; and an active layer formed on the first insulation layer, the active layer including source and drain electrodes, and channel and offset regions formed between the source and drain electrodes, the offset region being defined by the offset spacer.

The present invention can be also achieved in a whole or in parts by a semiconductor device, comprising: a first gate electrode formed on a substrate and having a first prescribed shape; a second gate electrode formed on the first gate electrode and having a second prescribed shape; an offset spacer formed in peripheral side surfaces of the second gate electrode and on the first gate electrode, the offset spacer having a height lower than the second gate electrode; an insulation film formed over the substrate, first and second gate electrodes and the offset spacer; and an active layer formed on the insulation film, the active layer including source and drain electrodes, and channel and offset regions formed between the source and drain electrodes, the offset region being defined by the offset spacer.

Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein:

FIG. 1 is a sectional view illustrating a background art thin film transistor;

FIGS. 2A to 2D are sectional views of the process steps illustrating a method for fabricating a thin film transistor of FIG. 1;

FIG. 3 is a perspective sectional view illustrating a thin film transistor according to a preferred embodiment of the present invention;

FIGS. 4A to 4G are sectional views of the process steps illustrating a method for fabricating a thin film transistor of FIG. 3; and

FIGS. 5A to 5C are sectional views illustrating the process steps of forming an insulating layer in accordance with another preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 3 is a cross-sectional illustration of a thin film transistor in accordance with a preferred embodiment of the present invention. A first semiconductor layer 42 is formed on a substrate 41, and a column or cylindrical shaped second semiconductor layer 46 formed on the first semiconductor layer 42, which has a concave shape in its lower portion serve as a gate electrode. Insulating layer patterns 45 a and 45 b are formed on a concave portion in the lower portion of the second semiconductor layer 46. A gate insulating film 47 is formed on an entire surface of the substrate 41 including the insulating patterns 45 a and 45 b, the second semiconductor layer 46 and the first semiconductor layer 42. An active layer 48 is formed on the gate insulating film 47, which includes source and drain electrodes 48 a and 48 b, a channel region I, and an offset region II.

The insulating layer patterns 45 a and 45 b are formed to surround the lower portion of the column shaped second semiconductor layer 46. The first and second semiconductor layer 46 are used as a gate electrode. The source electrode 48 a is formed in the active layer 48 at an upper portion of the second semiconductor layer 46. The drain electrode 48 b is formed in the active layer 48 at both sides of the first semiconductor layer 42. The offset region II is formed in the active layer 48 corresponding to any one of the insulating layer patterns 45 a and 45 b. The channel region I is formed in the active layer 48 between the offset region II and the source electrode 48 a.

A method for fabricating the thin film transistor according to the preferred embodiment of the present invention will be described with reference to FIGS. 4A to 4G. As shown in FIG. 4A, a first semiconductor layer 42 is formed in a predetermined portion on a substrate 41. The first semiconductor layer 42 is a polysilicon layer doped with a p type impurity ion. The first semiconductor layer 42 is patterned by photolithography. The substrate 41 is formed of an insulating material. Alternatively, a substrate in which an insulating layer is formed on a semiconductor substrate is used as the substrate 41.

As shown in FIG. 4B, a first insulating layer 43 is formed on the substrate 41 including the first semiconductor layer 42. The first insulating layer 43 is selectively removed to partially expose a top surface of the first semiconductor layer 42, so that a contact hole 44 is formed.

Subsequently, as shown in FIG. 4C, a second insulating layer 45 is formed on the first insulating layer 43 including the contact hole 44. The second insulating layer 45 is then etched back to form insulating layer patterns 45 a and 45 b at sides of the first insulating layer 43 in the contact hole 44, as shown in FIG. 4D. The first insulating layer 43 can be a silicon nitride layer, while the second insulating layer 45 can be a silicon oxide layer having etching selectivity different from that of the first insulating layer 43.

The insulating layer patterns 45 a and 45 b are not formed at the entire sides of the first insulating layer 43 in the contact hole 44 but formed with a predetermined height from the bottom at the sides of the first insulating layer 43. This is the reason why the second insulating layer 45 is overetched due to its etching selectivity being different from that of the first insulating layer 43.

The insulating layer patterns 45 a and 45 b may be formed on ends of the first semiconductor layer 42 or at a predetermined distance toward inner sides from ends of the first semiconductor layer 42. The insulating layer patterns 45 a and 45 b include sidewall spacers. Instead of the sidewall spacers, the process steps of forming insulating layers are applicable to the insulating layer patterns 45 a and 45 b.

Alternative process steps of forming insulating layers will be described with reference to FIGS. 5A to 5C. As shown in FIG. 5A, an insulating layer is formed on the first semiconductor layer 42 and patterned by etching process to form insulating layers 45 c and 45 d on both ends of the first semiconductor layer 42.

As shown in FIG. 5B, a first insulating layer 43 is formed on an entire surface including the insulating layers 45 c and 45 d. The first insulating layer 43 is selectively removed so as not to expose the substrate at both sides of the first semiconductor layer 42. Thus, a surface of the first semiconductor layer 42 is exposed.

Subsequently, as shown in FIG. 5C, a second semiconductor layer 46 which is used as a gate electrode can be formed on the exposed insulating layers 45 c and 45 d including the exposed first semiconductor layer 42.

Meanwhile, in the first embodiment of the present invention, as shown in FIG. 4D, the semiconductor layer 46 is plugged in the contact hole 44 in which the insulating layer patterns 45 a and 45 b are formed. In other words, a polysilicon layer is formed on the first insulating layer 43 including the contact hole 44 and then selectively removed to remain only in the contact hole 44.

Subsequently, as shown in FIG. 4E, the first insulating layer 43 is removed except for the first semiconductor layer 42, the insulating layer patterns 45 a and 45 b and the semiconductor layer 46. A gate insulating film 47 is then deposited on the entire surface of the substrate 41 including the second semiconductor layer 46 and the first semiconductor layer 42.

As shown in FIG. 4F, an active layer 48 is formed on the gate insulating film 47. The active layer 48 is a polysilicon layer which will include source and drain electrodes, a channel region and an offset region.

As shown in FIG. 4G, p conductivity type impurity ions for source and drain are implanted into the active layer 48. The active layer 48 over the second semiconductor layer 46 becomes a source electrode 48 a and the active layer 48 at both sides of the first semiconductor layer 42 becomes a drain electrode 48 b or vice versa.

The channel region I and the offset region ions are formed between the source electrode 48 a and the drain electrode 48 b. The channel region I is formed in the active layer 48 corresponding to one side of the second semiconductor layer 46 except for the insulating layer patterns. The offset region II is formed between the channel region I and the drain electrode 48 b.

At this time, since the second semiconductor layer 46 has a cylindrical shape, the channel region I is formed to surround an upper portion of the second semiconductor layer 46. The offset region II is defined by a height of any one of the insulating layer patterns 45 a and 45 b. In other words, the offset region II is defined by the factor as to how the second insulating layer is overetched to form the insulating layer patterns 45 a and 45 b.

As aforementioned, the thin film transistor and the method for fabricating the same according to the present invention have various advantages. Since the channel region is formed to surround the cylindrical shaped gate electrode, it is possible to increase the width of the channel region. Since the offset region is controlled by the gate voltage, it is possible to increase on-current, thereby improving on/off characteristic of the thin film transistor.

The foregoing embodiments are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. 

What is claimed is:
 1. A method for fabricating a thin film transistor comprising: forming a first semiconductor layer on a substrate; forming insulating layer patterns directly on both ends of the first semiconductor layer; forming a second semiconductor layer on the first semiconductor layer and the insulating layer patterns wherein the width of the second semiconductor layer which is formed adjacent to the insulating layer patterns is less than the width of the second semiconductor layer which is not formed adjacent to the insulating layer patterns; forming a gate insulating film on the first and second semiconductor layers and the insulating layer patterns; and forming an active layer on the gate insulating film.
 2. The method for fabricating a thin film transistor of claim 1, wherein the forming of the insulating layer patterns includes: forming a first insulating layer on an entire surface of the substrate including the first semiconductor layer; exposing a surface of the first semiconductor layer by selectively removing the first insulating layer to form a contact hole; forming a second insulating layer on the entire surface of the substrate including the contact hole; and etching back the second insulating layer.
 3. The method for fabricating a thin film transistor of claim 2, wherein the first insulating layer is a silicon nitride layer, and the second insulating layer is a silicon oxide layer having etching selectivity different from that of the first insulating layer.
 4. The method for fabricating a thin film transistor of claim 1, wherein the insulating layer patterns are formed with a predetermined height from the bottom at sides of the contact hole.
 5. The method for fabricating a thin film transistor of claim 2, wherein the height of the insulating layer patterns is determined by the amount of etching back of the second insulating layer.
 6. The method for fabricating a thin film transistor of claim 1, wherein the active layer corresponding to the source and drain regions is doped with p type impurity ions.
 7. The method for fabricating a thin film transistor of claim 1, wherein the active layer corresponding to a channel region and an offset region is not doped with any impurity ion.
 8. The method for fabricating a thin film transistor of claim 6, wherein the source region is formed in the active layer corresponding to an upper portion of the second semiconductor layer, and the drain region is formed in the active layer corresponding to both sides of the first semiconductor layer.
 9. The method for fabricating a thin film transistor of claim 7, wherein the channel region is formed in the active layer corresponding to sides of the second semiconductor layer except for the insulating layer patterns, and the offset region is formed in the active layer adjacent to the insulating layer patterns, wherein the height of the offset region corresponds to the height of the insulating layer patterns.
 10. The method for fabricating a thin film transistor of claim 1, wherein the insulating layer patterns include sidewall spacers and/or insulating layers.
 11. The method for fabricating a thin film transistor of claim 10, wherein the forming of the insulating layers includes: depositing an insulating material on the first semiconductor layer; and patterning the insulating material by photolithography to remain on ends of the first semiconductor layer at a predetermined width.
 12. The method for fabricating a thin film transistor of claim 11, wherein the forming of the insulating layer further includes: forming a second insulating layer on the entire surface of the substrate including the insulating layers and selectively removing the second insulating layer so as not to expose the substrate at both sides of the first semiconductor layer; and forming a third semiconductor layer on the exposed insulating layers including the exposed first semiconductor layer.
 13. The method for fabricating a thin film transistor of claim 1, wherein the gate insulating film is formed directly adjacent to portions of the substrate, the first and second semiconductor layers and the insulating patterns.
 14. The method for fabricating a thin film transistor of claim 1, wherein the insulating layer patterns are formed on peripheral side surfaces of the second semiconductor layer and wherein the height of the insulating layer patterns is lower than the height of the second semiconductor layer.
 15. The method for fabricating a thin film transistor of claim 1, wherein the width of the first semiconductor layer is wider than the second semiconductor layer.
 16. A method for fabricating a thin film transistor, comprising: forming a first semiconductor layer on a substrate; forming a first insulating layer on the substrate and the first semiconductor layer; forming a contact hole in the first insulating layer by selectively removing a portion of the first insulating layer to expose a portion of the first semiconductor layer; forming a second insulating layer on the first insulating layer and the first semiconductor layer; selectively removing portions of the second insulating layer to form insulating layer patterns on the surfaces of the peripheral corners of the first insulating layer and the first semiconductor layer; forming a second semiconductor layer in the contact hole on the first semiconductor layer, the insulating layer patterns and the first insulating layer, wherein the width of the second semiconductor layer which is formed adjacent to the insulating layer patterns is less than the width of the second semiconductor layer which is not formed adjacent to the insulating layer patterns; removing the first insulating layer; forming a gate insulating film on the substrate, the first semiconductor layer, the insulating patterns and the second semiconductor layer; forming an active layer on the gate insulating film, and forming a source and a drain region in the active layer.
 17. The method for fabricating a thin film transistor of claim 16, wherein the formation of the source and drain regions comprises: doping the horizontal surface of the active layer above the first semiconductor layer and the horizontal and vertical surfaces of the active layer.
 18. A method of fabricating a thin film transistor, comprising: forming a first semiconductor layer on a substrate; forming insulating layer patterns on the first semiconductor layer to a predetermined height and width; forming a second semiconductor layer adjacent to the first semiconductor layer and the insulating layer patterns, wherein the height of the second semiconductor layer is higher than the predetermined height of the insulating layer patterns, and wherein the width of the second semiconductor layer which is formed adjacent to the insulating layer patterns is less than the width of the second semiconductor layer which is not formed adjacent to the insulating layer patterns; forming a gate insulating layer on the second semiconductor layer; forming an active layer on the gate insulating layer; and forming source and drain regions in the active layer.
 19. The method for fabricating a thin film transistor of claim 18, wherein the gate insulating layer is formed over the substrate, the first and second semiconductor layers and the insulating layer patterns.
 20. The method for fabricating a thin film transistor of claim 18, wherein the source region is formed by ion implanting a dopant into an upper portion of the active layer above the first semiconductor layer and the drain region is formed concurrently with the source region by ion implanting a dopant into a lower portion of the active layer on both sides of the first semiconductor layer. 